In a pump of the type employed herein, a solenoid armature reciprocates to displace a diaphragm a distance commensurate with the length of the armature stroke. Since the diaphragm is disposed between suction and discharge ball valves, compression and relaxation of the diaphragm serves to pump process fluid therethrough.
A suitable metering pump which may be controlled by the supply voltage compensation circuit of the present invention is the 45 Series Chempulse.RTM. Electronic Pump of Wallace & Tiernan, Belleville, N.J.
The advent of inexpensive electronic power switching devices has fostered the popularity of solenoid driven metering pumps which are capable of delivering accurate and predetermined amounts of fluid. Such pumps readily lend themselves to proportioning applications where an external signal is sometimes used to trigger a solenoid stroke in direct proportion to another fluid flow for precise dosing.
Precise dosage or metering is characteristic of a metering pump which operates by moving a diaphragm in and out of the cavity by energizing and de-energizing an electromagnetic oil-filled solenoid in distinct pulses. Such a metering pump will typically pulse four to one hundred times per minute.
The force with which the diaphragm is pushed into the pump cavity varies over a single pulse from zero to some maximum value, and is a function of electrical current in the solenoid. The maximum piston force directly sets the maximum pumping pressure for the pump. This maximum pumping pressure is ideally maintained at a constant value, selected for a particular pump's construction and application. The maximum solenoid current, and hence the maximum pumping pressure, varies as the voltage applied across the solenoid windings varies with line voltage. These line voltage variations are normal.
In present designs, the presence of these voltage variations requires the power applied to the solenoid be sufficient to deliver the metered amount of fluid against rated back pressure at the lowest of operating line or supply voltages. At higher voltages, the energy and hence force delivered by the solenoid increases substantially. This energy increase however does not improve pump performance, but (a) causes the pump to run hotter resulting in the possible need for associated ventilating equipment, and (b) unnecessarily raises the solenoid driving force which increases the pump's output pressure to higher than normal operating pressures. Particularly in those applications where the pump's discharge piping path may have been terminated by the inadvertent closing of a valve, pressures may reach the burst pressure of the pipe with concomitant harm and/or danger to associated equipment and nearby personnel.
The present invention provides means for compensating the solenoid current, and hence, resulting pump pressure, for changes in supply voltage, i.e., to maintain substantially constant maximum pump pressure as supply voltage varies over its normal range.